Marta Silvia Maria Mantovani

Magmatism and continental break-up in the South Atlantic: high precision40Ar-39Ar geochronology

A detailed ArAr study of the Parana-Etendeka continental flood basalts (CFB) has been undertaken using laser spot analysis. Data provide information not only on the age of the samples but also on the variability of the non-radiogenic Ar component and state of alteration. The results indicate the Parana-Etendeka CFB were erupted over 10 million years between 137 and 127 Ma providing a minimum overall eruption rate of ∼ 0.1 km3 yr−1. This is an order of magnitude less than that previously proposed for this, the Deccan and Siberian CFB provinces but it is similar to estimates for Hawaii, Iceland and the Columbia River CFB. In detail, the new ArAr analyses indicate that magmatism within the Parana-Etendeka province migrated from NW to SE prior to and during the opening of the South Atlantic, providing an explanation for the asymmetry of the CFB lavas about the South Atlantic. Moreover, chemically defined magma types were erupted at different times in different places, and so within the Paranathey may not be used as reliable chronostratigraphic units. Rather, such magmatic units may reflect the extent of compositionally distinct source regions in the uppermost mantle, and indicate that partial melting took place over a wide area under the ParanaBasin. The NW-SE migration of the onset of magmatism might be interpreted as a plume trace, but the inferred rate of movement is 3 times faster than that inferred from subsequent magmatism on the Rio Grande Rise. It is argued that the onset of magmatism reflected extensional tectonics normal to the Ponta Grossa dyke swarm. Given that magmatism preceded the main phase of rifting, and that extension across the Ponta Grossa was moderate, current plume models have difficulty in predicting sufficient melting within the asthenosphere.

Chemical stratigraphy of the Paraná lavas (South America): classification of magma types and their spatial distribution

A new classification scheme has been developed to assign the lava flows of the Paraná continental flood basalt province (South America) into geochemically distinct magma types, with six basaltic major and trace element abundances and/or ratios. By mapping out the spatial distribution of these magma types within the lava sequences, it has been possible to determine the internal stratigraphy of the lava pile on a regional scale. Previous studies on road profiles traversing the well-exposed coastal Serra Geral escarpment of southern Brazil are summarised together with results from some new sampled sections. More widespread stratigraphical investigations of the Paraná lavas have been hampered by the lack of sufficient topographic relief and the cover of sedimentary rocks. However, access to drill-core chippings from nine boreholes in the central Paraná region has provided a unique opportunity to investigate the stratigraphy of the otherwise inaccessible deeper levels of the lava pile and to map out stratigraphic variations in three dimensions. The borehole samples have indicated cated a stacking of units of different magma types all overlapping towards the north, which suggests that the main locus of magmatism moved northwards with time within the Paraná basin. This migration could be related to the northward propagation of rifting during the initiation of the South Atlantic Ocean. Maps of the surface distribution of samples of each magma type show a pattern consistent with the stratigraphy inferred from the boreholes, although suggesting that the shift in magmatism may have been towards the northwest. On the basis of geochemical similarities between magma types and their inferred stratigraphical relationships, it is proposed that the Paraná can be divided into two principal magmatic centres: (1) an older one in the south, comprising the Gramado, Esmeralda and Urubici magma types; and (2) a younger one, developed about 750 km to the north, formed by the Pitanga, Paranapanema and Ribeira magma types.

3-D, 40AR-39AR GEOCHRONOLOGY IN THE PARANA CONTINENTAL FLOOD BASALT PROVINCE

New 40Ar39Ar analyses of borehole samples from the Parana-Etendeka continental flood basalt province provide critical evidence of eruption rates in unexposed regions. When combined with surface samples, the new data clarify the duration of this major volcanic province. New ages from both surface and borehole samples confirm a 10–12 million year duration of magmatism, which post-dates the Tithonian mass extinction, and provide a unique 3-dimensional picture of the spatial and temporal eruption of the lava pile. Chemically defined magma types are diachronous and the onset of magmatism occurred 500–1000 km inland, migrating southeast towards the incipient South Atlantic ocean. Calculated eruption rates increased from 0.03 km3 yr−1 between 138 and 135 Ma, through 0.13 km3 yr−1 between 135 and 133 Ma, to 0.21 km3 yr−1 between 133 and 131 Ma, supporting the notion that the rate of magmatism increased with time and proximity to the developing South Atlantic. After 131 Ma the continental eruption rates dropped dramatically to 0.01 km3 yr−1, presumably because rifting had occurred, and eruption centred in the oceanic Rio Grande Rise with eruption rates of around 0.3–0.5 km3 yr−1. Given the available constraints on the potential temperature of the plume, thickness of the continental lithosphere and degrees of extension through time, predicted melt production rates within the plume seriously underestimate the calculated eruption rates on the continent. However, the eruption rates can be reconciled if melting occurred by conductive heating of volatile-enriched mantle. The diachronous nature of the magma types supports the idea that melting occurred over a wide area and that the different magma types reflect different source regions rather than the temporal evolution of magmas from a single source. Given that 10 million years is appropriate to the timescales for conductive heating above the mantle plume, it is suggested that these distinct source regions were located within the lithospheric mantle, consistent with geochemical studies.

Mantle plumes and flood-basalt stratigraphy in the Paraná, South America

Geochemical studies of the Parana continental flood basalts in Brazil have led to the recognition of distinct magma types, which have been used to infer the internal stratigraphy of the lava pile. The overstepping of stratigraphic units toward the north and a similar compositional change in sills within the underlying Parana basin sedimentary rocks imply that the site of magnatism migrated ∼750 km toward the north during this volcanic event. This has important implications for recent plume-related models that have argued whether fithospheric rifting must accompany the presence of a mantle plume in order to generate continental flood-basalt magmatism. It is difficult to ascribe the magnitude and direction of the shift in the locus of the Parana magmatism to movement of the Brazilian lithosphere relative to the underlying Tristan mantle plume. Instead, it is suggested that the observed lava distribution and internal structure of the Parana-Etendeka flood-basalt province were imposed by the intracacies of the rifting process during the initial opening of the South Atlantic Ocean.

Tectonic controls on magmatism associated with continental break-up: an example from the Parana-Etendeka Province

The high- and low-Ti basalts of the Parana–Etendeka province were primarily derived from old, trace element-enriched source regions in the lithospheric mantle, and they are associated with dyke swarms of different orientations. These swarms appear to reflect different amounts of extension, and it is inferred that the high- and low-Ti magma types were characterised by different melt generation rates of ∼0.15 km3 yr−1 and ∼0.4 km3 yr−1, respectively [Stewart et al. Earth Planet. Sci. Lett. 143 (1996) 95–109]. There is probably a gap of ∼2 Myr between the end of the main phase of CFB magmatism and the oldest rocks on the adjacent ocean floor. A simple numerical model has been used to constrain the amounts and rates of melt generated from the continental lithosphere and asthenosphere under finite duration extension. Melting in the mantle is assumed to be controlled by the dry peridotite solidus in the asthenosphere and the hydrous (0.2% H2O) peridotite solidus in the lithosphere. For a maximum β of 4 and a duration of extension of 10 Myr, the derivation of melt from the asthenosphere by dry peridotite melting depends primarily on potential temperature (Tp) and is relatively insensitive to the thickness of the MBL, while the converse is the case for melt derived from the lithosphere by hydrous peridotite melting. For a Tp of 1450±50°C inferred from the crustal thickness estimates along the Rio Grande Rise and Walvis Ridge, the model successfully generates 2–4 km of lithosphere-derived melt before producing significant volumes of asthenosphere-derived melt. It is concluded that increases of melt volume with time can be generated by decompression melting of the mantle lithosphere. Critically, in areas of significant melt generation within the mantle lithosphere during extension and break-up, there is likely to be a gap in the volcanic record between the end of melt generation in the lithosphere and the onset of melting in the underlying asthenosphere. No such gap is present if all melts are generated within the mantle plume, and thus these models may in principle be tested in the geologic record.

Denudation, fission track analysis and the long-term evolution of passive margin topography: application to the southeast Brazilian margin

A detailed transect of fission track data is combined with present day elevation to constrain the evolution of topography across the continental margin of southeast Brazil since break-up. The fission track data show clearly that rocks on the lowlying coastal plain have been exhumed from temperatures of >110–120°C over the last 100 Ma, while the basalt capping the escarpment has not been at temperatures >50°C since its eruption ∼130 Ma. The fission track data are broadly consistent with a model of scarp retreat. Modelling the denudation history and associated isostatic rebound highlights the sensitivity of the predictions to the assumed effective flexural rigidity of the continental lithosphere. Estimates of denudation based on modelling the topography alone range from 2 to 6 km and the amount of denudation possible before elevation reaches sea-level is reduced for higher flexural rigidities (1023–25 NM). Consequently high geothermal gradients (45–60°C/km) are required to predict the fission track data satisfactorily. The greater inferred levels of denudation for flexural rigidities of 1019–21 Nm allow the fission track data to be predicted adequately with gradients of 15–30°C, values similar to present day estimates. An appropriate value for the flexural rigidity needs to be determined independently before we can confidently constrain the contributions of long term tectonic uplift and erosional rebound to the present day elevation.

The Paranapanema Lithospheric Block: Its Importance for Proterozoic (Rodinia, Gondwana) Supercontinent Theories.

This work considers the tectonics of the southeastern portion of the South American Platform based on new geological and geophysical grounds. For the last decade, only three (Amazonic, Sao Francisco and La Plata) of the many other cratonic blocks have been attributed/remarked to the South America portion for most of the usual Rodinia reconstitutions. The possibility of the existence of other blocks has rarely been mentioned. The postulation of the presence of a considerable Paleoproterozoic (pre-Brasiliano) fragment as part of Parana Basin basement is highly probable. In order to infer the basement structure of Parana Basin, previous to the sedimentation process, an isostatic modeling was applied to a large-scale gravity survey looking to correlate topographic and gravity anomalies caused by sub-surface loads. The Bouguer anomaly obtained from the gravity survey represents the crustal contribution of crystalline basement, in addition to the sedimentary and volcanic layers of the basin. Following the isostatic modeling and the basin load stripping, the residual anomaly allows observing similarities between the basement gravity signature and outcropping units. Besides, the stress pattern of the two earlier events obtained through the back stripping analysis presents a geographically coincident maximum, and a new E-SE high emerging for the second event, suggesting continuous change of the stress field as a precursor for South American plate rotation. The evident correlation between gravity highs and main attenuation suggests the presence of some pre-existing suture zones. The weakened lithosphere during Ordovician and Carboniferous provided the magma conduits to form in Early Cretaceous tectonic stress field pattern. The resultant mosaic of gravity blocks and the main faults site give support to the presence of this cratonic Proterozoic unit, here on referred to as the Paranapanema Block, which had been neglected in most of the models reported for the reconstruction of Gondwana (and Rodinia).

Radon anomalies and volcanic eruptions

Abstract A well-documented case of 222Rn anomaly preceding the eruptive activity of Karymsky volcano (Kamchatka) was recently reported in the literature. Stimulated by this example, we have attempted to utilize the available data on radon emanation from rocks, its solubility, and its circulation in waters to discuss how a 222Rn anomaly can be produced by magma approaching the surface. It is shown that the most likely process of radon release is the flushing of gases through pore fluids. Heating of extensive fracture surfaces by high-temperature gases may also be important. In order to survive in detectable amounts after moving distances greater than a few meters, radon must be transported by fast-moving fluids, such as those rising toward the surface through a fracture or in the ascending limbs of fluids convecting in very porous rocks under high temperature gradients. The pattern observed at Karymsky volcano is interpreted on the basis of these relations.

Tidal gravity anomalies as a tool to measure rheological properties of the continental lithosphere: application to the South American Plate

Characterization of the bulk physical properties of individual tectonic units is important for understanding the dynamics of continental evolution. One important parameter is effective elastic thickness (Te), a measurement of the flexural strength of the lithosphere. Te is traditionally estimated by examination of the transfer function between gravity and topography. Sparse gravity data coverage limits the application of this method in South America. Instead, we use an empirical correlation (as defined by data from Australia and by the world databank) between tidal gravity anomalies and Te to estimate Te for tectonic units of South America. Our results are consistent with independent determinations of Te in several sub-regions of South America. Although the empirical correlation appears to be quite strong, further research needs to be done to develop a physical theory for the connection between gravity tide anomalies (which sample an essentially instantaneous rheological response of the Earth) and Te (which measures the rheological response of the Earth at geological time scales).

Chapter 7.1 The Paranapanema Lithospheric Block: Its Nature and Role in the Accretion of Gondwana ☆

Abstract The SW Gondwanan Parana Basin in Brazil is developed on a gneissic-granitic terrane differing from the other basement rocks. Its geologic and geophysical characteristics indicate the existence of a distinct continental lithosphere segment, the ‘Paranapanema Block’. Deep boreholes show a predominantly granitic composition. It acted as an upper plate during Neoproterozoic subduction processes. Zones of plate interaction mark the external boundaries of this portion of the basin, characterised by continental magmatic arcs and post-collision structures. Gravimetric data corroborate the geological observations. Well-defined gradients delineate its contour. Presence and location of its northern portion were confirmed by an MT survey. Small differences in seismic velocity suggested its cratonic nature. According to geochemistry, Sr and Pb isotopes, two major sources, divide the area into northern and southern segments. Recent studies point to the existence of two lithospheric blocks, one of which is the Paranapanema.